The xylenes, para-xylene, meta-xylene and ortho-xylene, are important to intermediates that find wide and varied application in chemical syntheses. Para-xylene upon oxidation yields terephthalic acid that is used in the manufacture of synthetic textile fibers and resins. Meta-xylene is used in the manufacture of plasticizers, azo dyes, wood preservers, etc. Ortho-xylene is feedstock for phthalic anhydride production.
Xylene isomers from catalytic reforming or other sources generally do not match demand proportions as chemical intermediates. Further, xylene isomers are generally present with ethylbenzene, which is difficult to separate or to convert. Para-xylene in particular is a major chemical intermediate with rapidly growing demand, but amounts to only 20 to 25% of a typical C8 aromatics stream. Adjustment of isomer ratio to demand can be effected by combining xylene-isomer recovery, such as adsorption for para-xylene recovery, with isomerization to yield an additional quantity of the desired isomer. Isomerization converts a non equilibrium mixture of the xylene isomers that is lean in the desired xylene isomer to a mixture approaching equilibrium concentrations.
Various catalysts and processes have been developed to effect xylene isomerization. In selecting appropriate technology, it is desirable to run the isomerization process as close to equilibrium as practical in order to maximize the para-xylene yield; however, associated with this is a greater cyclic C8 loss due to side reactions. The approach to equilibrium that is used is an optimized compromise between high C8 cyclic loss at high conversion (i.e., very close approach to equilibrium) and high utility costs due to the large recycle rate of unconverted C8 aromatics. Catalysts thus are evaluated on the basis of a favorable balance of activity, selectivity and stability.
Catalysts containing molecular sieves have become prominent for xylene isomerization in the past quarter-century or so. U.S. Pat. No. 3,856,872, for example, teaches xylene isomerization and ethylbenzene conversion with a catalyst containing ZSM-5 (MFI-type), ZSM-12 (MTW-type (IUPAC Commission on Zeolitic Nomenclature)), or ZSM-21 zeolite. U.S. Pat. No. 4,899,011 discloses isomerization of C8 aromatics using two zeolites such as ZSM-5 with different crystal sizes, each of which is associated with a strong hydrogenation metal. U.S. Pat. No. 4,939,110 discloses a catalyst for isomerization using two metals and a pentasil zeolite, which includes ZSM-12 (MTW-type) zeolite. U.S. Pat. No. 6,222,086 and U.S. Pat. No. 6,576,581 disclose a dual catalyst system for aromatics isomerization using at least one non-zeolitic molecular sieve and one zeolitic aluminosilicate. U.S. Pat. No. 6,448,459 discloses a liquid phase isomerization stage and a gas phase isomerization stage with EUO-type zeolite.
U.S. Pat. No. 4,962,258 discloses a process for liquid phase xylene isomerization over gallium-containing, crystalline silicate molecular sieves as an improvement over aluminosilicate zeolites ZSM-5, ZSM-12 (MTW-type), and ZSM-21 as shown in U.S. Pat. No. 3,856,871. The '258 patent refers to borosilicate work, as exemplified in U.S. Pat. No. 4,268,420, and to zeolites of the large pore type such as faujasite or mordenite. U.S. Pat. No. 5,744,673 discloses an isomerization process using beta zeolite and exemplifies the use of gas phase conditions with hydrogen.
U.S. Pat. No. 5,763,720 discloses a gas phase C9 aromatics transalkylation process with a treated MTW-type or alternatively with a treated beta zeolite, both with a hydrogenation metal component; U.S. Pat. No. 5,942,651 further discloses a two zeolite system with the first zeolite from U.S. Pat. No. 5,763,720 combined with a second zeolite with smaller pores such as ZSM-5. A two zeolite catalyst system for transalkylation was also disclosed in U.S. Pat. No. 5,789,641 with a first catalyst of mordenite and a second catalyst of mazzite. Other processes have referred to zeolite beta in the context of ethylbenzene production. U.S. Pat. No. 4,891,458 discloses a process for alkylation or transalkylation of an aromatic hydrocarbon, such as benzene, with an olefin alkylating agent or polyalkyl aromatic hydrocarbon transalkylating agent, under at least partial liquid phase conditions over zeolite beta. U.S. Pat. No. 5,030,786 discloses a dehydration process to reduce the water level for a mono-alkyl-benzene production process based on zeolite beta or zeolite Y. U.S. Pat. No. 5,750,814 teaches that the use of beta in a process for ethylbenzene production, via alkylation, which actually minimizes xylene production (see column 3, line 27). U.S. Pat. No. 5,811,612 discloses that diethylbenzene can be transalkylated with benzene to produce ethylbenzene. U.S. Pat. No. 6,440,886 discloses a surface-modified zeolite beta by treating a templated native zeolite with an acid prior to template-removal calcination.